The specification of this application relates to wireless communication networks including techniques for identifying mobile stations by base stations for transmission of data packets.
Wireless communication systems use a network of base stations to communicate with wireless devices registered for services in the systems. Each of the base stations and wireless devices is a radio transceiver which transmits and receives radio signals modulated with communication information which can include, among others, data such as voice data and other data content. In a wireless network, the wireless service to a geographic area is provided by dividing the area into radio cells and each radio cell can be further divided into two or more cell sectors. Base stations conceptually locate at the center of respective cells of their coverage and transmit information to a mobile station via forward link or downlink radio signals. Mobile stations transmit information to their serving base stations via reverse link or uplink radio signals. The radio signals between the base stations and mobile stations can be modulated to include voice or data traffic signals or both.
FIG. 1 illustrates one example of a wireless communication system which provides wireless services wireless devices or mobile stations situated within a geographic region 100. The region 100 is divided into a number of radio cells 110, conceptually represented by a hexagon in a honeycomb pattern. In practice, however, each cell may have an irregular shape, depending on various factors including the terrain surrounding the cell and traffic density. Each cell may be further divided into two or more sectors. One particular wireless communication system designed for high speed packet data services is 1xEV-DO, which is also known as High Date Rate (HDR) or High Rate Packet Data (HRPD) system. 1xEV-DO has been standardized as C.S0024 in the international standard group Third Generation Project Partnership Two (3GPP2) and has been published as IS-856 Revision 0 and Revision A standards in the United States.
In 1xEV-DO system, a mobile station 132, 134, which is also known as the access terminal (AT), determines and reports the data rate that can be supported on the forward link in the Data Rate Control (DRC) message. The base station 120, which is also known as the access network (AN), selects one Physical Layer packet for forward link transmission at a particular time slot, based on the DRC messages received from various mobile stations. The Physical Layer packet may be given more than one time slot for transmission. In this case, the transmit slots of a Physical Layer packet are separated by three intervening slots, during which the slots of other Physical Layer packets can be transmitted. If a positive acknowledgement (ACK) is received on the reverse link ACK Channel before all of the allocated slots have been transmitted, the remaining un-transmitted slots will not be transmitted and the next allocated slot may be used for the first slot of a new Physical Layer packet transmission. This technique is known as Hybrid Automatic Repeat Request (HARQ).
In order to identify a target mobile station to which a forward data packet is directed, the base station transmits a preamble on the in-phase branch of the complex signal (I-branch) before transmission of the forward data packet. Meanwhile, no signals are transmitted on the quadrature branch of the complex signal (Q-branch). The preamble contains a repetition of a 32 chip bi-orthogonal sequence as in 15-856 Revision 0 standard, or repetition of 64 chip bi-orthogonal sequence as in 15-856 Revision A standard. The 32-chip bi-orthogonal sequence is defined in terms of the 32-ary Walsh functions and their bit-by-bit complements byWi/232 for i=0, 2, . . . , 62  (1) W(i-1)/232 for i=1, 3, . . . , 63  (2)where i=0, 1, . . . , 63 is the Media Access Control (MAC) Index value and Wi32 is the bit-by-bit complement of the 32-chip Walsh function of order i. The MAC Index is a number, which is assigned by the base station for identifying a mobile station in the system. Some MAC Index values are used as common values to all mobile stations for the purpose to identify the Control Channel, Broadcast, or Multi-User Packet transmissions. The 64-chip bi-orthogonal sequence is defined in terms of the 64-ary Walsh functions and their bit-by-bit complements byWi/264 for i=0, 2, 126  (3) W(i-1)/264 for i=1, 3, . . . 127  (4)where i=0, 1, . . . , 127 is the MAC Index value and Wi64 is the bit-by-bit complement of the 64-chip Walsh function of order i. The repetition of 32-chip bi-orthogonal sequence is a subset of the 64-chip bi-orthogonal sequence, as Walsh functions can be generated by means of the following recursive procedure:
                                          H            1                    =          0                ,                                  ⁢                              H            2                    =                                                    0                                            0                                                                    0                                            1                                                    ,                                  ⁢                              H            4                    =                                                    0                                            0                                            0                                            0                                                                    0                                            1                                            0                                            1                                                                    0                                            0                                            1                                            1                                                                    0                                            1                                            1                                            0                                                    ,                                  ⁢                              H                          2              ⁢              N                                =                                                                      H                  N                                                                              H                  N                                                                                                      H                  N                                                                                                  H                    _                                    N                                                                    ,                            (        5        )            where N is a power of 2 and HN denotes the binary complement of HN. Therefore, IS-856 Revision A standard doubles the MAC Index numbers while supporting the legacy mobile stations that comply with the IS-856 Revision 0 standard in an IS-856 Revision A network. The length of the preamble is variable from 64 chips to 1024 chips, depending on the data packet format.
The IS-856 Revision A standard provides a Multi-User Packet (MUP) to support real-time or near real-time services such as Voice over Internet Protocol (VoIP). A MUP is a data packet having one or more Security Layer packets addressed to different mobile stations. The base station transmits a preamble with a common MAC Index value for each Multi-User Packet to all mobile stations that support the Multi-User Packet. The individual MAC Index values for the target mobile stations of the Multi-User Packet are embedded in the MAC Header in each MAC Layer packet. A mobile station decodes the Multi-User Packet to determine whether it is one of the target mobile stations specified in the Multi-User Packet. A maximum of 128 MAC Index values can be supported in an IS-856 Revision A system.